In the worst river pollution incident ever to hit Brazil, a million-gallon oil spill is threatening drinking water, wildlife, and farmland along at least 25 miles of the Iguacu River in the southern part of the nation. The spill, which began Sunday afternoon, was caused by a ruptured pipe at a refinery of the government-owned Petrobras oil company. The company, which was also responsible for a January oil spill in a bay off Rio de Janeiro, will be fined at least $28 million. Dead, oil-covered fish, birds, and mammals have been washing up along the river’s banks. Workers have been racing to contain the spill, and government officials think they can keep the oil from spreading beyond Brazil’s borders into Argentina, Paraguay, and Uruguay.